Superconducting wires have enabled high current density in a conductor, which has enabled high field magnets. This in turn has enabled magnetic resonance imaging (MRI) of increasing resolution and clarity, a goal for all medical diagnosis. Traditionally, the wire of choice has been niobium titanium (NbTi), developed several years ago for the intended use of achieving high magnetic fields in high energy physics applications. The emergence of MRI has since dominated the market for this product for a number of reasons. First, NbTi is ductile and can undergo considerable strain (˜1%) before failing, thereby greatly simplifying manufacturing. Second, successive lengths can be easily joined by compressing the bare exposed NbTi filaments together and securing them in a pot of solder, and in particular, in a pot of low temperature solder. Despite all the benefits offered by NbTi wire, it has a single weakness that greatly affects its desirability as a MRI superconductor. It quenches (undergoes transition into normal electrical conducting mode) easily. Micro joules of energy can drive local temperatures from its stable operating temperature of 4.2° K. (liquid helium) to its operating limit of ˜5.5° K., depending on current density and background field.
Wire joints are necessary because a MRI must be constructed of several segments of superconducting wire. A low resistance joint is also useful for other superconducting applications such as superconducting fault current limiters, motors, generators, etc. With the current technology using NbTi wire, this is relatively simple. A copper stabilizer is etched away from the very ductile and acid resistant NbTi and the individual filaments are compressed together, coiled and secured in a container of Wood's metal, which as one skilled in the art will recognize, is a fusible alloy that becomes liquid at approximately 70° C. (158° F.), and is a eutectic alloy of bismuth, lead, tin, and cadmium with the following percentage by weight: 50% Bi, 26.7% Pb, 13.3% Sn and 10% Cd. The joint is effectively persistent having an electrical resistance of <10−12 Ohm, which is sufficient for normal MRI operation and maintenance.
MRIs have a device that permits the superconducting magnet to be autonomous. This persistent switch is made up of a small coil of superconductor co-wound with a resistive heater, which is actuated external to the magnet cryostat. It is electrically connected in parallel with the superconducting coil and a charging power supply. When the heater is on, it renders the persistent switch “normal” and more resistive than the superconducting magnet. A few volts applied across the leads cause current to flow from the high-current charging power supply into the superconducting magnet. When the target field and concomitant current are reached, the heater is turned off making the persistent switch superconducting and allowing the current to flow through it instead of the charging power supply. The charging leads are removed leaving the superconducting magnet with a persistent magnetic field, which will remain until the low electrical resistance joints and inherent low resistance of the superconducting wire in the magnet dissipate the field to an unacceptable level. At this point, the process is repeated recharging the coil. Persistent joints may be required in two places; in and out of a persistent switch and between wire coil segments that form a solenoid or other electromagnet shape made up of a plurality of wire segments.
Magnesium diboride (MgB2) was shown to be superconducting in 2001 and was made into filamented wire form shortly thereafter. With its critical temperature of 39° K., it is highly temperature tolerant and not sensitive to local low energy inputs such as cracks in epoxy potting material. However, joining the ends of successive wire lengths presents several issues. MgB2 has a low strain tolerance, 0.4% maximum and a working strain limit of 0.2%. This is not as sensitive as certain competitor high temperature superconducting (HTS) materials, such as BSCCO and YBCO, but it requires special consideration when attempting to place MgB2, particularly reacted MgB2, into direct contact with MgB2, a necessary condition for joint persistence and applicability to MRI magnets. The limiting factor is inextricably tied to the required construction of MgB2 wire.
MgB2 wire has only a few constituent materials, generally four in number, each providing an essential function. The MgB2 itself is frequently contained in a sheath of niobium (Nb) or iron (Fe). However, titanium (Ti), tantalum (Ta), and nickel (Ni) can be used to prevent the degrading effect of any surrounding copper (Cu) or copper alloy contamination during heat treatment, which can take place in a range of between approximately 1 and 1200 minutes in duration and a temperature of approximately 550° to 900° C. In a preferred embodiment, such heat treatment may take place in approximately 20 minutes at approximately 650° C. The Cu provides the essential stability for MgB2 while it is carrying current in the superconducting mode. At the moment of incipient quench, the current leaves the MgB2 and enters the Cu or Cu alloy. In its optimum condition, the Cu carries the current temporarily and either heats ohmically or cools, allowing the current to reenter the MgB2. Around the outside of the wire is a strong, ductile material used to facilitate drawing to the required size and shape. Although there are other candidate materials, to date, the material of choice continues to be Ni—Cu alloys, such as in some embodiments, Monel® 400 (MONEL is a trademark of Special Metals Corporation, headquartered in New Hartford, N.Y., U.S.A.) It provides the ductility necessary to draw the Cu and Nb or Fe without failure while providing electrical and thermal properties suitable for stability when wound into a magnet.
In order to place the MgB2 from one wire into direct contact with the MgB2 in another wire, the surrounding material must be stripped away. This leaves the Nb, Fe, or other sheath materials and the MgB2, which has a low strain tolerance vulnerable to fracture when compressed against its neighbor. This required exposure is exacerbated by the fact that any means of removing the Nb (note that the Ni—Cu alloy and Cu can be easily removed by chemical etching) destroys the MgB2. If mechanical means are done improperly, cracks can occur thereby breaking the reacted MgB2 and losing electrical continuity, while chemical means may etch away the MgB2 faster than it does Nb, a refractory metal. The challenge is to expose a sufficiently large surface area of MgB2 that is still supported by the Nb or Fe sheath to allow either direct contact of the MgB2 from each of the two wires and/or addition of reacted MgB2, and/or un-reacted Mg+B powder, and/or unreacted stoichiometric, or slightly off-stoichiometric, Mg+2B or Mg+2B+dopants for subsequent sintering.
A key problem to address is the manufacturability of a persistent joint. The goal is to be able to wind coils—there may be 5 to 8 in a single MRI solenoid shaped magnet—and join the ends to form a continuous wire connection. It is unlikely that pressing wires or filaments together will effect a persistent joint regardless how exposed the filaments are, because the Nb and Fe sheaths may be resistive, particularly in a background magnetic field. The possibilities include in-situ heat treating of the Mg+B, the stoichiometric or off-stoichiometric Mg+2B, or Mg+2B+one or more dopants with the components being in powder, semi-solid, or solid form, in intimate contact with the reacted MgB2 wire ends. It may be necessary to clamp the two ends of adjacent coils into a single fixture, fill it with Mg+B, stoichiometric or off-stoichiometric Mg+2B, or Mg+2B+ one or more dopants and heat treat it without damaging the wires, i.e., without breaking the MgB2 filaments.
Methods exist in current literature for both wire and joints for mixing stoichiometrically 53% by weight Mg and 47% by weight B powders with and without dopants to form wires as well as a bridge between two exposed ends of reacted MgB2 superconductor to fabricate a joint. These disclosures fail in achieving the best method for forming the aforementioned superconducting bridge between the two ends of previously reacted MgB2 wire. This less than desirable result stems from the fact that as the stoichiometric, or slightly non-stoichiometric mixed powders, Mg+2B forms into MgB2, there is a decrease in density, thus, the MgB2 that is forming in the joint is shrinking (pulling away) from the interface of the prepared MgB2 reacted wires and the joint being formed. Because of this effect, there is the potential to not obtain the ideal superconducting properties through the wire-to-joint interface. The MgB2 joint material forms as the Mg diffuses into the adjacent B particles. Thus, when a stoichiometric mixture (or one nearly stoichiometric) is used, the density of a Mg+2B powder mixture when converted to MgB2 by heating, the density of the MgB2 plus void volume at the interface of the reacted wires is less than the original un-reacted individual Mg+2B powders plus void.
The process of MgB2 formation is one where the Mg, which surrounds B particles in a well mixed, nearly stoichiometric powder mixture, begins as precursors. In this specification, the term “superconducting precursor” and/or “precursor” shall mean those elements, compounds or mixtures containing both Mg and B, which upon suitable reaction processes, have the capacity to form electrically superconducting compounds. Precursor or superconducting precursors may also contain variable amounts of already formed MgB2. The Mg diffuses into the B to form MgB2, and voids (porosity) are present where the Mg was previously located. In a macro sense, this densely packed precursor mixture becomes less dense with the depletion of the magnesium (Mg). This density reduction is approximately 20% less than the original packed volume. As this MgB2 formation takes place and the packed volume of the precursor powder mix becomes less dense, there is a tensile strain placed on the interconnecting superconducting bonds at the reacted MgB2 wire surfaces and new MgB2 material being formed in the joint. The joint is bounded by the confining fixed volume of the joint housing, and the fixed reacted wires on both sides of the joint. In this way, the shrinkage from the formation of the MgB2 may result in breaking these bonds and diminishing the current carrying capacity of the MgB2, formed to bridge the gap between two ends of reacted MgB2 superconducting wire.
With the alarming news that helium supplies worldwide are dwindling fast, there is a scramble among MRI suppliers to develop conduction cooled magnets that use a cryocooler to replace the currently used liquid helium bath cooled closed systems. The added expense of initial filling and required refilling at any time there is a quench or when maintenance is required diminishes the marketability of any MRI system. Conversion from NbTi superconductor to MgB2 superconductor in MRIs is a logical step in making the transition to a cryocooler based conduction cooled magnet since it takes advantage of the greater temperature tolerance of the MgB2. This allows greater flexibility in designing the heat sink that maintains the coil temperature as a larger gradient can be tolerated. When operating below ˜10° K., MgB2 is well below its critical transition level, which minimizes the likelihood of a quench. This effect is particularly important for the persistent joint, which usually has a lower critical current than the wire. The persistent joint should be mounted on the heat sink as close as practicable to the cryocooler cold-foot and in the lowest field zone around the magnet.
One of the problems to be solved in joining superconducting wires is to connect the actual superconducting filaments with one another. Prior attempts to do so with MgB2 have failed because the sheath material surrounding the MgB2 is too electrically resistive to simply overlap, as is the standard method with NbTi currently in use. The actual MgB2 must be in electrically superconducting contact between the two wires being joined. Ideally, the reacted MgB2 needs to be exposed so that the mix of Mg+2B, MgB2 or combinations of these, when reacted, form a superconducting bridge between the two wire ends.
Present disclosed methods to produce MgB2 joint formation uses near stoichiometric mixture of powder form Mg+2B as the joint material. This produces MgB2 formation that is less dense when compared to the original Mg+2B mixture. Thus, the newly formed MgB2 tries to pull away from the prepared MgB2 wire surfaces. The joint materials and arrangements in multiple embodiments of the instant invention allow for adjustment, that is, increase in joint volume of formed, so as to create a condition of positive pressure between the formed MgB2 and the prepared MgB2 wire surfaces. If parameters are adjusted properly, cracking due to over pressure and lower superconducting properties due to lower volume of materials in the joint when MgB2 is formed are minimized.